Many wireless communication networks are based on a contention protocol such as the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) contention protocol. This protocol is used when a wireless station (e.g. a WLAN node) which is about to transmit communications, listens to the network (senses the carrier) before transmitting the communications and waits for an opportunity during which it may transmit the communications. The wireless station will not transmit as long as a packet is either currently detected in the air or air time was reserved by another wireless station for a predefined duration of time (virtual carrier sense). According to IEEE 802.11, the wireless station awaits a random period of time and then, if the air resource becomes available during that period, transmits the communications. If the receiver gets the frame intact, it sends back an ACK or a block ACK message (according to the request from the transmitter) to the sender, indicating that the packet has arrived. This protocol is mainly used for multiple-access and significantly reduces the possibility of contention, i.e. that two or more wireless stations (e.g. WLAN nodes) will be transmitting at the same time.
When deploying such wireless networks in a long range configuration, there is a known problem which is referred to in the art as the “hidden node” problem (also known as the “hidden terminal” problem). This problem addresses cases where a plurality of wireless stations (also referred to as mobile subscribers, mobile terminals, mobile stations etc.) which are connected to the same access point, do not “hear” each other, thus cannot detect the carrier of each other and consequently two or more stations may transmit with high probability at the same time thereby causing a collision. As a result, the Access Point (AP) is unable to detect neither one of the two or more transmitted signals, so that the two or more stations will select a larger contention window and try to retransmit after a random period—in which case collision might still happen again. As the same scenario might repeat itself, the stations will continue increasing the contention window until it is high enough to substantially reduce the probability of contention, and the final result would be low air efficiency. This problem increases along with the increase in the number of wireless stations.
Some suggestions were made in the art to address the “hidden node” problem. One such solution is described in our co-pending patent application No. 214002 filed on Jul. 10, 2011.
The IEEE 802.11 protocol for example, uses a feature called RTS/CTS mechanism. According to this mechanism, each wireless station that is about to transmit a communication, sends a short Request-To-Send (referred to hereinafter as “RTS”) packet to its respective access point and the access point returns a Clear-To-Send (referred to hereinafter as “CTS”) packet. Both packets include the expected duration of the wireless transaction. All the other wireless stations detect the RTS packet or the CTS packet and create virtual carrier sense, which prevents them from accessing the air while the data transaction is still being transmitted.
Although this solution reduces the “hidden node” problem, still, it does not completely solve it. The reason being that in case the RTS/CTS mechanism is used, RTS packets may still collide with each other, and the outcome is that these transmitting stations will have to retransmit their RTS packets.
The problem further increases in cases where the stations are deployed at different ranges from the access point or have high power differences at the access point input. In these cases, the transmissions sent by one station may be received at a much higher power than those transmitted by another (e.g. farther) station. In such cases, when collisions occur between packets in general and RTS packets specifically, there is a high probability that the packet arriving with the higher power will not fail, and although a collision has occurred, the stronger packet will be received while the weaker one will fail.
The adverse effect of this problem is felt significantly in long range deployments, where transmissions sent by some stations are received with low SNR while transmissions sent from others are received with high SNR, and consequently, the stations whose transmissions are received with high SNR might cause starvation to stations whose transmissions are received with low SNR.
As will be appreciated by those skilled in the art, the colliding RTS packets scenarios may also occur in indoor or short range conditions. For example, in a case where a high number of stations transmit long packets, RTS packets may be used to decrease the penalty on collision. In this case, instead of retransmitting the long packet every time a collision occurs, only RTS packets may be retransmitted, till a period of time has been established, during which a certain STA may transmit the long packets without experiencing any collisions from other STAs.